This invention relates generally to optical gain devices and, more specifically, to optical gain devices using stimulated Raman scattering.
An optical gain medium is a device that, when provided with pumping energy, increases the amplitude of a desired optical signal. Optical gain media may be constructed using optical fiber, and used as fiber lasers or fiber-based optical amplifiers. One form of optical gain mechanism known in the art is based on stimulated Raman scattering (SRS). In such a device, optical pumping energy is injected into an optical fiber medium. The optical pumping energy, via SRS, allows for a transfer of optical power to a wavelength longer than the pumping wavelength, due to the excitation of a vibrational mode in the medium that provides gain at the longer wavelength.
The longer wavelengths to which optical power is transferred may be predetermined relative to the wavelength of the pumping energy. Each wavelength shift is referred to as a xe2x80x9cStokesxe2x80x9d shift and, since it is a known amount, the resulting wavelength may be selected by proper selection of the pumping wavelength. Some Raman gain media use only a single wavelength shift to produce optical energy at a desired wavelength. In another type of Raman device, overlapping resonant cavities are constructed for a number of shifted wavelengths all based on the same initial pumping wavelength. That is, the shifted wavelength resulting from the pumping wavelength, referred to as the xe2x80x9cfirst Stokes orderxe2x80x9d is resonated within the gain medium, generating its own shifted wavelength that is a predetermined amount longer than the first order wavelength. This xe2x80x9csecond orderxe2x80x9d wavelength is, in turn, resonated within the cavity to generate a xe2x80x9cthird Stokes orderxe2x80x9d wavelength. By accurate selection of the different resonant reflectors and the initial pumping wavelength, a number of Stokes shifts may be used to generate an optical signal at a desired wavelength significantly longer than the pumping wavelength. This type of optical device is generally referred to as a xe2x80x9ccascaded Raman resonatorxe2x80x9d (CRR), and an example of such a device is described in U.S. Pat. No. 5,323,404.
In accordance with the present invention, an optical gain apparatus is provided that includes a gain medium that produces optical gain in response to optical energy at a pump wavelength. In a preferred embodiment, the gain medium provides gain via SRS, and the gain is therefore at a wavelength longer than the pump wavelength. A plurality of fused fiber couplers are used, including one coupled to an input port of the system and one coupled to an output port. The fused fiber couplers each have two signal paths on each of a first side and a second side. Each coupler is arranged such that signals directed to the coupler along the first and second signal paths exit the coupler along the third and fourth signal paths, and vice versa. The first coupler has its first path coupled to an input port and its second path coupled to a first optical side of the gain medium. The second coupler has its fourth path coupled to an output port and its third path coupled to a second optical side of the gain medium.
In the third and fourth signal paths of the first coupler are located input reflectors, each of which is reflective at an input wavelength. Thus, optical energy at the input wavelength that is directed into the first coupler along its first signal path is directed to both input reflectors and, as such, is directed back into the first coupler. These two signal portions are in phase only along the second signal path of the first coupler, and all of the reflected energy is therefore coupled along the second signal path toward the gain medium. Similarly, in the first and second signal paths of the second coupler are located output reflectors, each of which is reflective at an output wavelength. Thus, optical energy at the output wavelength that is directed from the gain medium into the third signal path of the second coupler is directed to both output reflectors and, as such, is directed back into the second coupler. The two signal portions reflected back to the second coupler are in phase only along the fourth signal path, and all of the reflected energy is therefore coupled toward the output port of the system. The optical energy not reflected by the output reflectors is coupled back toward the gain medium.
In the preferred embodiment, the optical gain apparatus is configured as an optical ring, such that the optical energy travels continuously through the ring in one direction until the output wavelength is reached, and the light is reflected by the output reflectors toward the system output port. Light not at the output wavelength bypasses the output reflectors and remains in the ring. In a preferred embodiment, the light that bypasses the output reflectors is coupled through two more fused fiber couplers. This light is then directed toward the input reflectors adjacent to the first coupler, but approaches from the side of the input reflectors opposite the first coupler. This light bypasses the input reflectors and, being at the correct relative phase, is all coupled through the first fused coupler, and out its second signal path toward the first optical side of the gain medium.
In one variation of the invention, a reflector is used in the ring structure that is reflective at the output wavelength. This reflector is positioned so that any light at the output wavelength that is traveling opposite to the desired ring direction is reflected back to the desired direction. In another variation, a plurality of reflectors are used in the ring, each of which has a narrow band reflectivity at a different Stokes order relative to the input wavelength. Thus, when the apparatus is functioning as a CRR, the different reflectors allow the wavelength at each different Stokes order to be selected with precision.
In an alternative embodiment of the invention, a ring configuration is used, but without the fused fiber couplers. The ring includes a Raman gain medium, and operates as a CRR. Wavelength selective couplers, preferably thin film type couplers, are used to couple input optical energy into the ring and to couple output optical energy out of the ring. In the preferred version of this embodiment, the input coupler has a cutoff wavelength, such that wavelengths significantly longer than that of the input optical energy are prevented from passing through the coupler, while the input wavelength passes through unimpeded. Similarly, the output coupler has a cutoff wavelength such that wavelengths significantly shorter than a desired output wavelength are prevented from passing through the coupler, while optical energy at the output wavelength passes out through the coupler to an output port. Shorter wavelengths are maintained in the ring, and continue propagating through the Raman gain medium. The couplers may be arranged such that input and output light is transmitted through the coupler filter elements into and out of the ring, respectively, or they may be arranged so that the input and output light is reflected by the coupler filter elements into and out of the ring. As in aforementioned embodiments, a reflector may be used at the wavelength of the output light to maintain a single direction of propagation through the ring. Likewise, one or more reflectors in the ring may be used to select intermediate Raman wavelengths at the various Stokes orders being used with the system, if the device is being used as a CRR.
In accordance with the invention, a single coupler is provided that accomplishes both the input and the output functions of a thin film type coupler embodiment. This coupler has a pair of graded index (GRIN) lenses separated by a filter element. The filter element includes two separate materials, each of which has a different wavelength filter function. In one embodiment of the invention, the materials are coatings on opposite surfaces of the filter element. A first one of the materials may have a xe2x80x9chigh-passxe2x80x9d type wavelength characteristic, such that wavelengths at or below a first cutoff wavelength coupled into a first optical path of the coupler are reflected into a second optical path by the first material, while longer wavelengths are transmitted through it. Similarly, the other material may have a xe2x80x9clow-passxe2x80x9d type wavelength characteristic, such that wavelengths at or above a second cutoff wavelength coupled into a third optical path of the coupler are reflected into a fourth optical path by the second material. Wavelengths below the second cutoff wavelength that enter along the third optical path are transmitted through the filter element to a desired optical path, such as the second optical path of the coupler. With this arrangement, the filter element has an overall reflectivity characteristic that allows optical energy between the first and second cutoff wavelengths to be trapped within an optical ring, continuously coupled between the third optical path and the second optical path. Meanwhile, optical energy with wavelengths at or below the first cutoff wavelength may be coupled into the ring using the reflectivity of the first material, and optical energy with wavelengths at or above the second cutoff wavelength may be coupled out of the ring using the reflectivity of the second material.